Multiple channel analog frequency division multiplexing for use in communicating multiple simultaneous information channels is well known in the art.
The two most common modes of carrier modulation used in such systems are amplitude modulation and frequency modulation. Normal amplitude modulation is double side band, which results in the creation of two identical side bands, frequency displaced on each side of the carrier frequency by an amount identical to the modulating frequency. The strength of the side bands, in such a system, is directly proportional to the amplitude of the modulating frequency. While double side band amplitude modulation is simple and inexpensive to implement, it also inherently generates certain disadvantages. The transmission occupies twice the band width of the highest modulating frequency, has no particular signal to noise advantage, as is exhibited in more complex transmission systems, and is subject to phase errors between the two side bands. Such modulation presents particular problems if the communication medium exhibits non-uniform phase characteristics at varying frequencies within the channel band width.
Frequency modulation for carrier multiplexed systems requires more complex and expensive equipment, and increases the signal-to-noise ratio only at the cost of wider occupied band width.
Amplitude modulation is often modified to provide single side band transmission to reduce the required band width for transmission, and involves the suppression of the carrier and one side band at the transmission point. The signal-to-noise ratio can be made correspondingly higher. However, the equipment is much more complex than with standard amplitude modulation, particularly because the carrier must be reinjected at the receiver.
Some of the complexity of single side band reception can be eliminated by transmitting both a single side band and the carrier, while suppressing the remaining side band. Such transmission, however, suffers high distortion in an environment where the carrier frequency at the receiver is subject to drift. In addition, such transmission requires an extremely accurate and costly transmission filter.
True vestigial side band transmission systems slightly attenuate the carrier frequency in comparison with a main side band, and substantially attenuate the remaining (vestigial) side band. Such systems allow some band width economy in transmission, and are used, for example, to transmit picture information for television broadcast. True vestigial side band systems, however, suffer from high amplitude distortion if the receiver tuner drifts so that the carrier frequency is not accurately placed within the receiver pass band. Furthermore, the detected signal suffers from high harmonic distortion at high modulation indices.
Single side band, single side band plus carrier, and true vestigial side band transmission have classically been utilized in transmission systems to reduce the amount of band width required for transmission, so that more communication channels may occupy the same transmission band width, while the disadvantages of these modulating techniques have been reduced or eliminated through the use of expensive, complex equipment.
However, in systems where equipment cost and complexity are of significant concern, it has been typical, in the prior art, to utilize double side band amplitude modulation and to limit overall system performance to a level consistent with this modulation. These limitations are a primary design consideration, for example, in multiple channel, carrier multiplexed telephone systems operating over extended wire pairs. Such transmission systems exhibit attenuation which increases with increasing frequency, such that higher frequency transmission necessitates either higher amplitude transmission or more complex repeaters placed at more frequent intervals. The use of frequent repeater amplifiers is costly, while the use of higher signal levels requires the transmission of greater power levels which, in turn, increases the cost and losses of the system. Thus, such communication systems are essentially bandwidth limited. In addition, these systems exhibit non-linear transmission medium phase characteristics at certain carrier channels as a consequency of the use of repeater amplifiers and extremely long cable runs, so that the effective usable band width and range of the systems must be compromised with one another to form a viable transmission network at reasonable cost.